In designing loudspeaker systems, there are two known methods for achieving a desired frequency response characteristic at low frequencies. The first method involves making changes to the physical parameters of the speaker system itself. For instance, a driver of pre-determined size is selected for installation in a speaker box of pre-determined size in order to obtain a desired frequency response. A problem with this first method is that the required driver and speaker box selections are often either physically impractical, or are not economically feasible. The second method involves the connection of an equalizing circuit in cascade with the amplifier that powers the loudspeaker. This offers the speaker designer greater freedom in selecting physical components by leaving the final response adjustments to the equalizing circuit.
Conventional equalizing circuits operate by boosting the lower part of the system response through the use of a simple resonating circuit, an underdamped high pass filter, or other known filters. One problem with such equalizing circuits is that the desired system frequency response is achieved with only limited accuracy. The equalizing circuit is only successful at approximating the desired frequency response, it does not succeed in precisely equalizing the response. Furthermore, the addition of such equalizing circuits to the loudspeaker system increases the total system order and can negatively influence the quality of the reproduced sound.
A possible solution to these problems is to utilize two state-variable, or two biquad filters. However, this approach leads to significantly more complex circuitry which increases costs, and further complicates the system.
What is needed is an equalizing circuit for accurately achieving a desired target frequency response for a loudspeaker system at low frequencies. Furthermore, there is a need for such a circuit to incorporate a relatively simple combination of components. Finally, there is a need for a method for designing such a circuit for a loudspeaker system of any order.